1. Field of the Invention
The present invention relates to porous materials and more particularly to a porous composition for use in an environment of variable pressure and/or variable temperature.
2. Description of the Related Art
The Payload Fairing (PLF) is the portion of a launch vehicle that encloses the payload, i.e., satellite. One purpose of the PLF is to protect the satellite from the loads and aerodynamic heating associated with launching the satellite into space. Another is that the PLF protects the satellite from contamination, as even the smallest particle of dust or debris could render the satellite useless. The PLF and all parts of the PLF must be designed so that substantially no particles, dust, or any other type of possible contamination would fall onto the payload.
In order to protect the PLF structure from the effects of aerodynamic heating, a thermal protection material is placed over the PLF structure, particularly over the nose on the front of the PLF, which receives the majority of aerodynamic heating. One such thermal protection material is known as MCC-1 (for NASA Marshall Convergent Coating). This material was developed by NASA for use on the Space Shuttle, but is also used by the Air Force on the PLF of the Titan IV launch vehicle. MCC-1 is applied robotically and requires the use of an ancillary material, to repair the MCC-1 and to fix application defects. To repair MCC-1 NASA uses what is known as K5NA, a mixture of epoxy and cork, that serves the NASA application on the solid rocket motors of the Space Shuttle well. But, the solid rocket boosters do not have the same contamination requirement as the Titan IV PLF. Testing showed that the NASA repair material (i.e., K5NA) would not meet the Titan IV PLF contamination-free requirement. It was demonstrated that small pieces of the NASA repair material would break off during flight. Since the portion of the PLF that needed the most thermal protection material was the forward nose cone of the PLF, i.e., directly in front of the payload, any particles coming off the repair material were almost certainly going to contaminate the payload.
It was found that xe2x80x9cpopped offxe2x80x9d small pieces of the repair material were the result of air being entrapped in the material during the process of mixing the ingredients. The aerodynamic heating of the material increased the pressure of the entrapped air, while at the same time the external air pressure was dropping to the vacuum of space. In addition, the heating reduced the strength of the repair material. As a result the repair material failed, leading to the generation of the xe2x80x9cpop offxe2x80x9d particles. MCC-1 did not suffer this same fate, because MCC-1 was permeable, allowing the entrapped air to escape without doing damage to the material. The inherent permeability of MCC-1 was the result of the fact that the material was sprayed onto the PLF. MCC-1 had air inside of the material, which increased its efficiency as a thermal protection material, but the air was not entrapped. MCC-1 was permeable, allowing any air in the material to escape as the local air pressure dropped, or the air expanded due to the aerodynamic heating. Because of the robotic application of MCC-1, it was not a suitable repair material, particularly at the launch site, located away from the factory at which MCC-1 was applied with special equipment.
It was found that pulling a vacuum on the NASA repair material after mixing the ingredients removed the larger air bubbles, but did not make the material permeable. As a result, when the repair material was used to cover MCC-1, the air entrapped in the MCC-1 pushed pieces of the repair material off. It was clear that a material that could easily be mixed, applied by hand or with a simple spray process, cured at ambient conditions (xcx9c50xc2x0 F. to 90xc2x0 F.), and was permeable, was required.
U.S. Pat. No. 4,204,899, issued to Walker et al., entitled xe2x80x9cCork-Resin Ablative Insulation for Complex Surfaces and Method of Applying the Same,xe2x80x9d discloses a cork-resin ablative insulation material, which may be applied to complex surfaces. The material is intended for broad coverage and is limited to resins that can be B-staged to form the thin, pliable cork sheet that is applied to the surface. It could not be used to fill in nicks, blemishes or other defects. Since it contains 20 to 60 weight percent resin, the cork content would be 40 to 80 weight percent. Such a large concentration of cork would not yield a material that could be worked by hand, or troweled onto a surface. Present applicants"" investigations have revealed that if more than 20 weight per cent cork (xcx9c90 volume percent due to the differences in density of the resin and cork) is used, the material will not hold together or bond to the substrate to be repaired, e.g., MCC-1.
U.S. Pat. No. 3,844,863, issued to Forsythe, et al., entitled xe2x80x9cRepair of Wooden Articles,xe2x80x9d discloses a method to repair defects and holes in wood. In a preferred composition, the comminuted cork comprises from 3 to 10 percent by weight of the mixture. Such low weight percentages of cork would not achieve the desired effect of a contamination-free thermal insulator in a variable temperature/variable pressure environment.
U.S. Pat. No. 4,031,059, issued to Strauss, entitled xe2x80x9cLow Density Ablator Compositions,xe2x80x9d discloses two types of highly filled, elastameric silicone-base ablative compositions whose densities range from about 0.2 g/cc to about 0.3 g/cc. One type is a carbon char-forming, high thermal efficiency ablator containing at least 92% by volume of low-density filler with a total filler-to-resin volumetric ratio of at least 16 to 1. The material""s core content is too high to allow its application in a hand-packing mode. It would have to be molded into final shape, obviating its ability to act as a repair material. The second type of material disclosed by Strauss is a silica char-forming, RF-transparent ablator containing at least 90% by volume of low-density filler with a total filler-to-resin volumetric ratio of at least 11 to 1. Again, this material would have to be molded into final shape, obviating its ability to act as a repair material.
U.S. Pat. No. 4,077,921, issued to Sharpe, et al., entitled: xe2x80x9cSprayable Low Density Ablator and Application Process,xe2x80x9d discloses a sprayable, low density ablative composition having 100 parts by weight of a mixture of 25-65% by weight of phenolic microballoons, 0-20% by weight of glass microballoons and 4-10% by weight of glass fibers, 25-45% by weight of an epoxy-modified polyurethane resin, 2-4% by weight of a bentonite dispersing aid and 1-2% by weight of an alcohol activator for the bentonite; b) 1-10 parts by weight of an aromatic amine curing agent; and c) 200-400 parts by weight of a solvent. These additives are insufficient to achieve permeability effects required to prevent xe2x80x9cpop-off.xe2x80x9d
U.S. Pat. Nos. 4,772,495 and 4,837,250, both entitled xe2x80x9cTrowelable Ablative Coating Composition and Method of Use,xe2x80x9d both issued to Headrick et al. disclose coating compositions which have insufficient quantities of cork particle to prevent pop-off.
U.S. Pat. No. 5,064,868, issued to Simpson et al., entitled xe2x80x9cSprayable Lightweight Ablative Coating,xe2x80x9d discloses another composition with insufficient quantity of cork to prevent pop-off in a variable pressure/variable temperature environment.
It is therefore a principal object of the present invention to meet the contamination-free requirements of the material to be used in a variable temperature and/or variable pressure environment.
Another object is to provide a material that could be either sprayed onto hardware or applied by hand.
Another object is to provide a composition, which can be applied in ambient conditions.
Yet another object is to provide a composition that is thermally insulative.
These and other objects are achieved by the present composition, which in its broadest aspects comprises a mixture of a binder and a particle material. The mixture has 50 to 90 volume percent of the particle material. The binder is curable in a temperature range of 50xc2x0 F. to 90xc2x0 F.
The volume percent of the present composition provides an inherent permeability in any conventional application method. It can be sprayed or applied by hand and yet it still maintains an intrinsic permeability.
It can be used as a primary material or a repair material where prevention of xe2x80x9cpop-offxe2x80x9d is important.
The material can also be used over another material that has air or another gas or even a liquid entrapped in it and let the gas/fluid out without causing damage to either material. Because the material is permeable, it can operate repeatedly in a variable pressure and/or variable temperature environment without being damaged due to the effects of the entrapped gas/fluid. Some applications require that these materials experience temperatures as high as 900xc2x0 F. for very short periods of time. One example would be use as a thermal protection material on a launch vehicle. Pressure ranges for the launch vehicle application would range from 1 to 0 atmospheres. Another application, e.g., underwater, could result in large pressure increases being applied to the material in the range of 1 to 40 atmospheres. Current generation materials would be limited to the level of such environments because the current generation materials are subject to damage by the entrapped air.
These and other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.